Considerable recent research has involved the development of optical amplifiers useful in optical communications. Typically, these amplifiers involve a waveguide formed in a glassy material (a material that has no long-range ordering and is characterized by an absence of Bragg peaks in X-ray diffraction and/or a glass transition observed in differential scanning calorimetry) with a rare earth dopant present in the waveguide core and with a region of lower refractive index surrounding the core. Generally, the glassy host material does not substantially affect the emission spectrum of the dopant, and the rare earth dopant material is chosen to have a spectral emission line corresponding to a wavelength at which optical communication is to be performed. For example, most long-haul optical communication is performed either at 1.3 .mu.m or 1.55 .mu.m. Optical devices that amplify signals at 1.3 .mu.m are described in U.S. Pat. No. 5,140,658 to Sunshine et al.
Optical amplifiers at 1.55 .mu.m have been demonstrated and are described in U.S. Pat. No. 5,119,460 to Bruce et al. These amplifiers involve a waveguide fiber having erbium, that emits at 1.52 to 1.56 .mu.m, present in the core at concentrations typically in the range 10 to 1000 parts per million. During operation of the amplifier, optical power at a wavelength 0.975 or 1.48 .mu.m is introduced into the waveguide core along with a signal at the 1.55 .mu.m wavelength. The optical power induces a transition in the erbium that populates a state, the .sup.4 I.sub.13/2 state, capable of stimulated emission around 1.55 .mu.m, and the signal induces this transition from the populated state. Thus, the output from the amplifier involves a signal at 1.55 .mu.m that has an intensity approaching that of the combined power and signal inputs. In his manner, an optical signal is amplified, in contrast to electrical amplification involving conversion of the optical signal to an electrical signal, followed by electrical amplification and another conversion back to an optical signal.
The concentration of the dopant affects the efficiency of the amplifier. Since the properties of the amplifier depend upon the absolute number of dopant atoms in the host material, the dopant concentration that is necessary for adequate performance depends upon the length of the device. For example, the dopant concentration in fiber amplifiers is much less than the dopant concentration in planar optical amplifiers, because fiber amplifiers are much longer than planar optical amplifiers. However, high dopant concentrations lead to concentration quenching of the luminescence from the dopant. If such quenching occurs, the amplifier gain is reduced and the amplifier performance is consequently degraded. Therefore, planar optical amplifiers that amplify signals at 1.55 .mu.m and that overcome the problems associated with high dopant concentration, and a process for making such planar optical waveguides, are sought.